The big questions that will come up when you try to get paid for your work will be either fair use or ownership. If someone uses your images without permission especially if they put their name on your images without your written consent the damages can be very large. Do not allow anyone to use your images without your permission but if they put their name on them you cannot just let it go. You need to fight that type of violation at all costs.
You can do a lot to prove ownership besides the registration of the work. Be smart and register your work. Also be smart and register the ones that are being used for commercial work (not work for hire) and ones that may get copied by others. Work for hires may be yours but that depends on your contract so take note of that before you register to make sure that they are yours.
There are many ways to prove ownership and I have mentioned them before but a simple one is to have a work flow that helps you do that. The way I work is that I shoot RAW and process it from there.
I shoot raw, 14 bit uncompressed; I transfer images to the computer via different programs from Lightroom, Bridge, Picassa and Nikon Transfer. My main program is now Lightroom 5. Once in the library I process or develop them using LR and saving or exporting them as tiff files while not damaging the NEF or raw file. NEF is Nikon’s extension for their raw files. If the images have more work to be done then they are processed in PS. The work is saved as a JPG if they are being used on the internet or other less demanding situations. I also save the original tiff file as another tiff if major changes are done. Yes it is a lot of copies but they prove a lot if needed.
I also many times upload the images to a photo site for storage. That puts the image and information in a specific time frame. The images are also stored on hard drives more than one time. I know it is redundant but trust me you will need that sometime. I failed to do that while transferring images from a couple of questionable hard drives. The single backup was taken. Enough said.
In our modern world of take what you want you may get into a problem with someone or someone’s attorney concerning proving ownership. It is not too hard to deal with if you have the original image as a raw file and a tiff file. Ask the other party to produce the raw image like yours. The jpg is easy to work around as it will have been modified by programs that may cost a very large amount of money. You also have file sizes, copyright information put into the images and many other sources. Uploading to a storage site helps as it will have dates, times and image information. Best is to do all. I have somewhere around a thousand images registered and more to do. It takes time but with your really good images or important ones it is well worth the effort.
I am not an attorney. These are my personal opinions and general information only. These are not legal advise and should not be used as such.